Cotangent is Reciprocal of Tangent
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Theorem
Let $\theta$ be an angle such that $\cos \theta \ne 0$ and $\sin \theta \ne 0$.
Then:
- $\cot \theta = \dfrac 1 {\tan \theta}$
where $\tan$ and $\cot$ mean tangent and cotangent respectively.
Proof
Let a point $P = \tuple {x, y}$ be placed in a cartesian plane with origin $O$ such that $OP$ forms an angle $\theta$ with the $x$-axis.
Then:
\(\ds \cot \theta\) | \(=\) | \(\ds \frac x y\) | Cotangent of Angle in Cartesian Plane | |||||||||||
\(\ds \) | \(=\) | \(\ds \frac 1 {y / x}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac 1 {\tan \theta}\) | Tangent of Angle in Cartesian Plane |
$\tan \theta$ is not defined when $\cos \theta = 0$, and $\cot \theta$ is not defined when $\sin \theta = 0$.
$\blacksquare$
Sources
- 1953: L. Harwood Clarke: A Note Book in Pure Mathematics ... (previous) ... (next): $\text V$. Trigonometry: Definitions of the ratios
- 1968: Murray R. Spiegel: Mathematical Handbook of Formulas and Tables ... (previous) ... (next): $\S 5$: Trigonometric Functions: $5.16$
- 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): Appendix $12$: Trigonometric formulae
- 2021: Richard Earl and James Nicholson: The Concise Oxford Dictionary of Mathematics (6th ed.) ... (previous) ... (next): Appendix $14$: Trigonometric formulae